JPH085396B2 - 4-wheel steering system for vehicles - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a four-wheel steering system for a vehicle, and more specifically, steering front wheels and rear wheels in response to operation of a steering wheel, The present invention relates to an improvement of a four-wheel steering system for a vehicle, which is configured to change a wheel turning ratio according to a turning ratio characteristic corresponding to a vehicle speed.

(Prior Art) A four-wheel steering system for a vehicle enhances the turning performance of the vehicle by controlling the steering ratios of the front and rear wheels to the opposite phase to set the steering characteristic to an oversteer tendency during a normal low-speed turn, thereby improving the turning performance of the vehicle and turning the high-speed turn. The steering characteristics are set in the same phase and the steering characteristics are set in the direction that strengthens the understeer, so that the running stability of the vehicle is enhanced, and the front wheels are set based on the predetermined steering ratio characteristics that are preset according to the vehicle speed. It is configured to change the steering angle of the rear wheels with respect to.

In a vehicle equipped with the four-wheel steering system, when the vehicle decelerates during turning, the steering ratio is reduced, that is, the rear wheels are in the opposite phase to the front wheels, resulting in oversteering. A tuck-in phenomenon that suddenly faces inward and a scooping phenomenon that the car body spins may occur. In order to prevent the occurrence of this tuck-in phenomenon or the like, conventionally, for example, as shown in Japanese Patent Laid-Open No. 60-85066, the steering ratio of the rear wheels to the front wheels is changed according to the vehicle speed only when the vehicle is in a substantially straight traveling state. When the vehicle is in a turning state, the steering angle of the rear wheels is kept constant, or when the vehicle speed changes abruptly as shown in JP-A-60-85067. Is configured to change the steering angle of the rear wheels with a predetermined delay by a delay circuit or the like.

However, in the former configuration, when decelerating in the high-speed turning state where the front wheel and the rear wheel are in the same phase, the rear wheel is kept in the same phase state even if the vehicle shifts to the low speed state, so that the turning property is However, if the front wheels and the rear wheels are accelerated in a low-speed traveling state in which the front wheels and the rear wheels are in opposite phases, the rear wheels are held in the opposite-phase state even after shifting to the high-speed state. There is a problem that the running stability of the vehicle cannot be improved, and the original characteristics of the four-wheel steering system cannot be exhibited in these states.

Further, in the latter configuration, when the vehicle is accelerated or decelerated, the steering angle of the rear wheels changes with a predetermined time delay. Was not obtained, and was insufficient in terms of running stability or turning ability.

(Object of the Invention) The present invention has been made based on the above technical background, and at the time of low speed, the turning ratio of the front and rear wheels can be controlled to the opposite phase to improve the turning performance of the vehicle, and at the same time, at high speed. The tack-in phenomenon of the vehicle body during transient operation such as deceleration at the time of turning while maintaining the function of the four-wheel steering device that can sometimes control the same phase of the steering ratio to improve the running stability of the vehicle An object of the present invention is to obtain a four-wheel steering system for a vehicle, which can easily prevent the occurrence of a biting phenomenon.

(Structure of the Invention) In order to achieve the above object, the present invention detects a vehicle speed in a four-wheel steering system for a vehicle including a front wheel steering mechanism that steers front wheels and a rear wheel steering mechanism that steers rear wheels. Control means for controlling the rear wheel steering mechanism based on the rear wheel steering characteristic that changes the steered angle of the rear wheels with respect to the front wheels according to the vehicle speed, the road surface friction coefficient detection means that detects the friction coefficient of the traveling road surface, The rear wheel steering control means corrects the rear wheel steering characteristics to the same phase side as the front wheels in accordance with the increase in the rate of change of the vehicle speed, and the rear wheel steering coefficient decreases as the road surface friction coefficient decreases. It is characterized in that it has rear-wheel steering characteristic correction means for correcting the steering characteristic correction amount so as to be further increased to the same phase side as the front wheels.

(Operation and Effect of the Invention) In the four-wheel steering system for a vehicle of the present invention configured as described above, the rear-wheel steering characteristic correcting unit causes the rear-wheel steering characteristics to be the same as those of the front wheels in accordance with an increase in the rate of change in vehicle speed. The correction is made to the phase side. As a result, preferable four-wheel steering of the vehicle can be achieved during transient operation such as acceleration or deceleration. Further, in the present invention, the rear-wheel steering characteristic correcting means corrects the correction amount of the rear-wheel steering characteristics such that the smaller the road surface friction coefficient is, the larger the correction amount becomes in the same phase as the front wheels. As a result, it is possible to prevent the occurrence of a tack-in phenomenon, which easily appears on a road surface having a low coefficient of friction (hereinafter referred to as a low-μ road), and to prevent the occurrence of a biting phenomenon, etc., and to perform stable traveling.

(Embodiment) Hereinafter, a four-wheel steering system for a vehicle according to a preferred embodiment of the present invention will be described with reference to the accompanying drawings.

1 and 2 show a schematic configuration of a four-wheel steering system for a vehicle according to an embodiment of the present invention, in which front wheels 1L, 1R and rear wheels are shown.
2L and 2R are front wheel steering mechanism 3 and rear wheel steering mechanism 12 respectively
Is supported by.

The front wheel steering mechanism 3 includes a pair of left and right knuckle arms 4
It is composed of L, 4R and tie rods 5L, 5R, and a relay rod 6 connecting the left and right tie rods 5L, 5R.
A steering wheel 10 is connected to the front wheel steering mechanism 3 via a rack and pinion type steering mechanism 7. That is, the steering mechanism 7 includes a rack 8 formed on the relay rod 6 and a steering shaft 11 having a steering wheel 10 connected to an upper end thereof and a pinion 9 engaging with the rack 8 attached to a lower end thereof. The left and right front wheels 1L and 1R are steered in accordance with the operation of 10.

On the other hand, the rear wheel steering mechanism 12 is similar to the front wheel steering mechanism 3 in that the left and right knuckle arms 13L and 13R and the tie rods are used.
The relay rod 15 connects 14L and 14R to each other, and further includes a hydraulic power steering mechanism 16.
The power steering mechanism 16 includes a power cylinder 17 fixed to the vehicle body and having the relay rod 15 as a piston rod. Inside the power cylinder 17, two hydraulic chambers are provided by a piston 17a integrally attached to the relay rod 15. It is divided into 17b and 17c, and these hydraulic chambers 17b and 17c are connected to a control valve 20 via pipes 18 and 19, respectively. Two pipes, an oil supply pipe 22 and an oil discharge pipe 23, which reach the reserve tank 21, are connected to the control valve 20, and a hydraulic pump 24 driven by an engine (not shown) is arranged in the oil supply pipe 22. It is set up.
The control valve 20 is of a known spool valve type, and is a cylindrical valve casing 20 integrally attached to the relay rod 15 via a connecting member 25.
a and a spool valve (not shown) fitted in the valve casing 20a, and supplies pressure oil from the hydraulic pump 24 to one hydraulic chamber 17b (17c) of the power cylinder 17 according to the movement of the spool valve. Then, the driving force for the relay rod 15 is assisted. In the power cylinder 17, return springs 17d and 17d for urging the relay rod 15 to a neutral position (positions where the steering angles θR of the rear wheels 2L and 2R are 0) are mounted.

The relay rod 6 of the front wheel steering mechanism 3 has a rack 26 at a position different from the rack 8 constituting the steering mechanism 7.
A pinion 27 attached to the front end of a rotary shaft 28 extending in the vehicle front-rear direction is meshed with the rack 26, and the rear end of the rotary shaft 28 is connected to the rear wheel steering mechanism via a steering ratio control mechanism 29. 12 connected.

As shown in FIG. 2, the turning ratio control mechanism 29 has a control rod 30 slidably held in the vehicle width direction with respect to the vehicle body. One end of the control rod 30 is one of the control valves 20. It is connected to the spool valve. The turning ratio control mechanism 29 has a U-shaped holder at the base end.
31 includes a swing arm 33 swingably supported via a support pin 32, and the holder 31 is rotated in a casing (not shown) fixed to the vehicle body in a direction orthogonal to the movement axis of the control rod 30. It is rotatably supported via a support shaft 35 having an axis. The support pin 32 of the swing arm 33 is located at the intersection of the two axes and extends in the direction orthogonal to the rotation axis. By rotating the holder 31 about the support shaft 35, the tip of the support pin 32 is supported. The inclination angle formed by the pin 32 and the moving axis of the control rod 30, that is, the rocking locus surface of the rocking arm 33 about the support pin 32 is formed with respect to a plane orthogonal to the moving axis (hereinafter referred to as a reference plane). It is designed to change the tilt angle.

Further, one end of a connecting rod 37 is connected to the tip end of the swing arm 33 via a ball joint 36, and the other end of the connecting rod 37 is connected to the other end of the control rod 30 via a ball joint 38. The control rod 30 is displaced in the vehicle width direction according to the displacement of the tip end of the swing arm 33 in the vehicle width direction.

The connecting rod 37 is slidably supported by the rotation imparting arm 40 via a ball joint 41 at a portion near the ball joint 36. The rotation imparting arm 40 is integrally provided with a large-diameter bevel gear 43 which is rotatably supported on the moving axis via a support shaft 42. As shown in FIG. A bevel gear 44 attached to the rear end of the rotation shaft 42 is meshed with the rotation shaft 42 to transmit the rotation of the steering wheel 10 to the rotation imparting arm 40. Therefore, the rotation imparting arm 40 and the connecting rod 37 rotate about the moving axis by an amount corresponding to the rotation angle of the steering wheel 10, and the swing arm 33 swings about the support pin 32 accordingly. If the axis of the support pin 32 coincides with the movement axis of the control rod 30, the ball joint 36 at the tip of the swing arm 33 only swings on the reference plane, and the control rod 30 remains stationary. However, if the axis line of the pin 32 tilts with respect to the movement axis line and the swing locus surface of the swing arm 33 deviates from the reference plane, the swing arm 33 swings around the pin 32. Due to the movement, the ball joint 36 is displaced in the vehicle width direction, and this displacement is transmitted to the control rod 30 via the connecting rod 37, and the control rod 30 moves along the movement axis, and the control rod 30 moves. And it is configured to actuate the spool valve of the roll valve 20. That is, the swing arm around the axis of the pin 32
Even if the swing angles of 33 are the same, the displacement of the control rod 30 in the left-right direction changes with the change of the tilt angle of the pin 32, that is, the rotation angle of the holder 31.

Then, in order to change the inclination angle of the support pin 32 with respect to the movement axis, that is, the inclination angle of the holder 31 with respect to the reference plane, the support shaft 35 of the holder 31 includes a sector gear 45 as a worm wheel, and the worm gear on the rotary shaft 46. 47 meshed. Further, a bevel gear 48 is attached to the rotary shaft 46, and a bevel gear 49 mounted on the output shaft 50a of the stepping motor 50 is meshed with the bevel gear 48, and the sector gear 45 is operated by operating the stepping motor 50. Is rotated to change the inclination angle of the holder 31 with respect to the reference plane to control the steering angles θR of the rear wheels 2L and 2R, and the sector gear 45
From the neutral position where the center line is perpendicular to the center axis of the rotary shaft 46 of the worm gear 47, when turning clockwise when viewed from above the vehicle body, the steering ratio is rear wheels 2L, 2R front wheels 1L, 1R It is configured to control in the same phase so as to face the same direction.

Further, in the casing supporting the holder 31, there are stopper members 51 on the opposite phase side and the same phase side, which are formed on the left and right sides of the sector gear 45 serving as the rotating member, and each of which includes a pin for restricting the rotation range of the sector gear 45, 52 is attached, and when the sector gear 45 rotates to the opposite phase side and the rotation angle from the neutral position becomes, for example, −17.5 °, the sector gear 45 contacts the phase changing side stopper member 51 and further Is restricted, and when the sector gear 45 rotates toward the same phase, when the rotation angle from the neutral position becomes, for example, 20 °, the sector gear 45 comes into contact with the stopper member 52 on the same phase side to move. It is configured to be regulated. The control position of the stepping motor 50 when the sector gear 45 contacts the stopper member 51 on the opposite phase side is set to the initial position. Reference numeral 39 is a rod stopper that restricts the maximum movement range of the relay rod 15 in the rear wheel steering mechanism 12.

As shown in FIG. 3, the stepping motor 50 is constructed so that its operation is controlled by an output from a control unit 100 with a built-in microcomputer.
A vehicle speed sensor 101 that detects a vehicle speed _V and outputs a vehicle speed signal S _V is connected to an input portion of the control unit 100. The control unit 100 receives the vehicle speed signal S _V from the vehicle speed sensor 101 and receives the reference step number calculation unit 1
In 02, the reference operation step number of the stepping motor 50 proportional to the value of the vehicle speed signal S _V is calculated, and the reference step number signal S _SO is output. This control unit 100
A driver 103 that receives the reference step number signal S _SO and operates the stepping motor 50 by the number of steps indicated by the signal S _SO is provided in the output section of the. By operating the stepping motor 50 according to the vehicle speed V,
The reference turning ratio characteristic indicated by the symbol C ₀ in FIG. 4 is obtained. According to this reference turning ratio characteristic C ₀ , the turning ratios of the front and rear wheels 1L, 2L (1R, 2R) change according to the vehicle speed, and when the vehicle speed is low, the turning performance of the vehicle is improved. The rear wheels 2L and 2R are steered in the opposite direction to the front wheels 1L and 1R, that is, in the opposite phase, and the steering ratio becomes negative, while the vehicle speed is about
When reaching 67 km / h, the steering ratio becomes zero and the front wheels 1
The steering angle θR of the rear wheels 2L and 2R is θR = regardless of the steering of L and 1R
0 is maintained and the vehicle is in the normal two-wheel steering state. In addition, when the vehicle speed is higher, the rear wheels 2L and 2R can be gripped more when cornering to improve driving stability.
2L and 2R are steered in the same direction as the front wheels 1L and 1R, that is, in the same phase, and the steered ratio is set to be positive.

The control unit 100 operates as a system power source, which is supplied from a vehicle-mounted battery when an ignition key switch (not shown) is turned on. The internal structure of the control unit 100 will be specifically described with reference to FIG. Then, the control unit 100 includes a CPU 106 as a control unit and a ROM 107 that stores predetermined control data, and the CPU 106 is operated by an output voltage from a constant voltage circuit 108 that keeps the battery voltage (12V) at a constant voltage of 5V. Then C
CPU runaway detection unit 109 that detects PU106 runaway, output voltage 4.5
It is reset by receiving each output from the voltage drop detection unit 110 that detects that the voltage has dropped below V and the power-on reset unit 111 that outputs a reset signal when the ON operation of the ignition key switch starts.

Further, the output signal of the vehicle speed sensor 101 is input to the integration filter 113 via the interface 112, and after the integration filter 113 removes chattering, the waveform shaping circuit 1
The signal waveform is shaped in 14 and supplied to the CPU 106.

Further, as described above, the control unit 100 has the stepping motor driver 103 which receives the output of the CPU 106 and drives the stepping motor 50, and receives the coerent down command signal from the CPU 106 to the stepping motor 50. Each phase has a current down section 116 that limits the output current from the battery power source to, for example, 100 mA while the motor 50 is not controlled (when the rotation of the motor output shaft 50a is stopped).

When the vehicle travels at a constant vehicle speed, if the four wheels are steered with the reference steering ratio characteristic C ₀ obtained as described above, a desirable traveling state can be obtained at each vehicle speed, but the vehicle speed increases rapidly. In the case of change, if steering is performed by using the standard turning ratio characteristic C ₀ as it is, there is a problem that the tack-in phenomenon is likely to occur and the push-in phenomenon occurs as described above.

Therefore, in the present invention, a rapid change in vehicle speed is detected, and the steering ratio characteristic to be used is corrected to the same phase side according to the degree of the change and the road surface μ, thereby preventing a timing phenomenon or the like. .

Therefore, in the above-described embodiment, the vehicle is provided with the front-rear G sensor 117 that detects the gravitational acceleration in the front-rear direction of the vehicle body, that is, the front-rear G, which is one of the factors indicating the rate of change of the vehicle speed, and outputs the G signal S _G. In addition to providing the stepping motor 50 in order to move the steering ratio characteristic to the same phase side,
The control unit 100 is provided with an additional step number calculation unit 118 and a target step number calculation unit 119 that operate in a number of steps larger than the reference number of steps determined based on only the vehicle speed V. As shown in FIG. 3A, the additional step number calculation unit 118 makes the number of additional steps when traveling on a relatively low μ road larger than that when traveling on a relatively high μ road for the same front and rear G. As described above, the map M indicating the relationship between the number of additional steps and the size of the front-rear G that differs depending on the μ value of the traveling road surface is stored in advance. As shown in FIG. 3A, this map M has characteristic lines L ₁ and L ₂ that differ depending on the μ value of the road surface on which the vehicle travels. The additional step number calculation unit 118 receives the road surface μ signal Sμ from the road surface μ sensor 120 connected thereto, and selects the characteristic line L ₁ or L ₂ to be used for control based on the signal Sμ. The additional step number calculation unit 118 also receives the G signal S _G from the front and rear G sensor 117, calculates the number of additional steps by comparing the G signal S _G with the selected characteristic line L _R or L _O , and adds Step number signal
Output S _SA . The additional step number calculation unit 118 and the reference step number calculation unit 102 are the target step number calculation unit 1
It is connected to 19 and this target step number calculation unit 119
Receives the reference step number signal S _SO and the additional step number signal S _SA from the above two calculation units 102 and 118, adds these signals S _SO and S _SA , and outputs the target step number signal corresponding to the target step number. Output S _SP . The driver 103 is connected to the target step number calculation unit 119, and the driver 103 receives the target step number signal S _SP from the calculation unit 119 and drives the motor 50 according to the signal S _SP . Thus, according to the size of the front and rear G, that is, the number of additional steps, as shown in FIG.
Corrected steering ratio characteristics C ₁ and C ₂ shifted from C _{0 in the} same phase direction are obtained.

Next, a signal processing procedure performed by the CPU 106 of the control unit 100 will be described. FIG. 6 shows a main routine of a signal processing program. The function of the steering ratio setting unit 104 is fulfilled by this routine. After starting by turning on the ignition key switch, the system is first initialized in step S ₁ , and in the next step S ₂ , the current step number MP of the stepping motor 50 is set to 580 and its target step number CP is set to 0. In addition to setting each, the flag F ₁ indicating execution of the motor position initialization control mode is set to F ₁ = 1. The target step number CP is CP = 0 at the control initial position of the stepping motor 50, that is, the position where the sector gear 45 is in contact with the antiphase stopper member 51 and the steering ratio is the maximum antiphase steering ratio. And the number of steps of the pulse signal input to the motor 50 when controlling the motor 50 to its target control position, and the current number of steps MP is the above control of the current control position of the motor 50. It shows the number of steps from the initial position. The flag F ₁ is set to F ₁ = 1 in the motor position initialization control mode in which the motor 50 is controlled to initialize its control position, but the steering ratio is controlled according to the vehicle speed. In the vehicle speed sensitive control mode, it is reset to F ₁ = 0.

Then, the procedure proceeds to step S _3, the flag F ₁ makes a determination of whether F ₁ = 1. When this determination is F ₁ = 1 (that is, when the position initialization control mode of the motor 50 is performed), the process proceeds to step S ₄ and the target number of steps for the motor 50.
CP is determined whether the current equal to the step number MP, the judgment returns directly to the step S ₃ when the CP ≠ MP.
Further, when the determination control position initializing the motor 50 in the CP = MP is completed, the process proceeds to step S _5, the target step number CP and the current step number MP of the motor 50 to CP = MP = 0, and stand While resetting F ₁ to F ₁ = 0,
After setting the flag F ₂ for identifying that the initialization of the control position of the motor 50 has been completed once to F ₂ = 1
Return to step S ₃ above.

On the other hand, when it is determined in step S ₃ that F ₁ = 1 is not satisfied and the motor 50 is controlled to change the steering ratio,
Proceed to Step S ₆ determines whether the traveling speed V detected by the vehicle speed sensor 101 is in the 0 (stopped state), when the determination is YES, further the flag F ₂ in step S ₇ F ₂ It is determined whether or not = 0. Then, it returns to the step S _3, control position initialization of the motor 50 at the time of stop of the F ₂ = 0 and the determination has been traveling speed V is 0 when the determination in step S ₇ is F ₂ ≠ 0 the if it is not running is confirmed, the flag F ₁ is set to F ₁ = 1 at step S _8, the motor in the next step S ₉
Back current step number MP and the target step number CP 50 in step S ₃ after setting the MP = 580, CP = 0 corresponding to the control initial position.

Further, in step S _6, the process proceeds to step S ₁₀ when the vehicle rather than traveling vehicle speed V 0 is determined to be running state, the reference step number BAS of the stepping motor 50 based on the read vehicle speed V, also read The number R of additional calculation steps is calculated based on the road surface μ and the front and rear GG. Next, at step S ₁₁ , the number R of calculated additional steps at this time is the number of target additional steps stored previously.
It is judged whether it is larger than OFF. If this judgment is NO,
Determines whether the steering angle theta _H of the steering wheel is close to zero in step S _12. When this determination is YES, and when the determination of R> OFF in step S ₁₁ is YES,
The target additional step count is set to OFF for the current additional calculation step count R
Memorize as. Thereafter, by adding the reference step number BAS and the target additional number of steps OFF, it calculates the target step number CP, and stores it, the both flag in the next step S ₁₅
After resetting both F ₁ and F ₂ to 0, the process returns to step S ₃ . By the above operation, as shown in FIG.
The target step number CP of the stepping motor 50 is calculated according to the turning ratio along the turning ratio characteristics C ₀ , C ₁ , and C ₂ .

FIG. 7 shows an interrupt routine for interrupting the main routine when the time set in the timer built in the CPU 106 has elapsed, and this routine serves as the stepping driver 103. . In the first interrupt routine, the target step number CP of the motor 50 at a first step S ₂₀ determines whether the current is equal to the step number MP. When the determination is CP = MP, that is, when the output of the pulse signal to the motor 50 to hold the motor 50 is unnecessary in its control position, the output current down command signal to the current down unit 116 proceeds to Step S ₂₁ it allows suppressing the heat value by lowering the voltage applied to the motor 50, and then returns to steps after interruption of the main routine after setting the timer for generating the next interrupt process in step S _22.

Further, when the decision in step S ₂₀ is CP ≠ MP, after releasing the output of the current down command signal for the current down unit 116 proceeds to step S _23,
The process proceeds to step S _24, the target step number CP of the motor 50
And the current step number MP are compared. By the time this determination is CP> MP, motor 5 proceeds to step S ₂₅
0 switching the excitation phase to move by one step in the same phase direction of the steering ratio, and then proceeds to step S ₂₂ after updating the current step number MP to MP ← MP + 1 in step S _26. On the other hand, when the decision in step S ₁₆ is CP <MP is switched the excitation phase as the motor 50 moves one step in the opposite phase direction of the steering ratio proceeds to step S _27, in step S ₂₈ currently it proceeds to step number MP to step S ₁₄ after updating the MP ← MP-1. By repeating the above, the stepping motor 50 is set to the target step number CP
The steering angle θR of the rear wheels 2L and 2R is set by the steering ratio according to the vehicle speed at that time and the magnitude of the front and rear G.

Although the mechanical turning ratio control mechanism 29 is used in the embodiment described above, the reference turning ratio characteristic shown in FIG. 4 is used.
C ₀ , the corrected turning ratio characteristics C ₁ , C ₂ , and C ₃ are stored in advance, and the steering wheel rotation angle θ _H , vehicle speed V, front and rear GG, and road surface μ are detected, and these θ _H and V are detected. , G may be illuminated with the characteristics C ₁ , C ₂ , C ₃ to determine the steering angle of the rear wheels in the control unit 100.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of a vehicle four-wheel steering system according to the present invention, FIG. 2 is a schematic perspective view of the steering system, and FIG. 3 is a block diagram showing the function of a control unit.
FIG. 3A is a diagram showing the relationship between the front-rear G used in the steering control of the present invention and the offset turning ratio, FIG. 4 is a characteristic diagram of vehicle speed and turning ratio, and FIG. FIG. 6 is a block diagram showing the configuration, FIG. 6 is a flowchart showing a main routine processed by the CPU in the control unit, and FIG. 7 is a flowchart showing an interrupt routine processed by the CPU. 1L, 1R …… Front wheel 2L, 2R …… Rear wheel 29 …… Turning ratio control mechanism 101 …… Reference step number calculation unit 117 …… Front and rear G sensor 118 …… Additional step number calculation unit 119 …… Target step number calculation Part 120: Road surface μ sensor.

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Claims (1)

[Claims] 1. A four-wheel steering system for a vehicle comprising a front-wheel steering mechanism for steering front wheels and a rear-wheel steering mechanism for steering rear wheels, and a vehicle speed detecting means for detecting a vehicle speed and a friction coefficient of a traveling road surface. And a rear-wheel steering control means for controlling the rear-wheel steering mechanism based on the rear-wheel steering characteristic that changes the steering angle of the rear wheels with respect to the front wheels according to the vehicle speed. The wheel steering control means corrects the rear wheel steering characteristics to the same phase side as the front wheels according to the increase in the rate of change of the vehicle speed, and the smaller the road surface friction coefficient, the further the correction amount of the rear wheel steering characteristics becomes to the same phase as the front wheels. A four-wheel steering system for a vehicle, comprising: a rear-wheel steering characteristic correcting unit that corrects the steering wheel so that the steering wheel becomes larger toward the side.